1 40. CONK LIN. [Vol. X 1 1 1 . 



is formed entirely from the four turret cells, or trochoblasts, as 

 Wilson appropriately calls them. Each of these cells divides 

 twice, forming in all sixteen cells, twelve of which compose the 

 prototroch. By the growth and division of these cells the tip 

 cells of the cross are forced into a position below the prototroch, 

 and are called by Wilson " post-trochal cells." Though they 

 take no part in forming the prototroch, they lie but one cell 

 below it, and may perhaps be considered as having been crowded 

 out of the trochal series by the rapid growth and early division 

 of the trochoblasts. 



Mead ('94) has found that in Amphitrite and Clymenella the 

 same cells form the prototroch as in Nereis. Each of these 

 trochoblasts divides twice, as in Nereis, and all sixteen of 

 these cells enter the prototroch. " Later the prototroch is 

 completed by the addition of nine more cells from the * second 

 generation of micromeres ' in quadrants A, B, and C respec- 

 tively." That these additional cells of the second quartette 

 are the same, at least in part, as the velar cells of Crepidula is 

 shown by the further statement made by Mead that " almost 

 the entire substance of a^-' (2ai) and c^' (2ci) enters into the 

 prototroch." 



Remembering the many points of difference between the 

 fully formed velum of the gasteropod and the prototroch of 

 the annelid, it is most interesting and instructive to find such 

 essential agreement in origin between the two. In fact, it 

 may be truly said that they are even more alike in origin than 

 in final structure. 



(d) The Shell Gland. — This characteristic molluscan organ 

 appears late in development in the case of Crepidula, Figs. 74 

 et seq. From its position on the mid line and at the posterior 

 end of the embryo it is probable that it comes from the group 

 of cells derived from 2d. 



It is formed in the first instance by the very rapid multipli- 

 cation of cells in a limited region of the ectoblast. These cells 

 are densely packed together, so that in surface views of the egg 

 only the nuclei can be recognized. At the centre of this pro- 

 liferating area the nuclei are smallest and most numerous, and 

 they grow successively larger from the centre toward the 



